I plan related studies of neural interactions at two levels of the visual system: the retina of the cat and the visual cortex of man. Retinal signal-processing will be investigated by studying responses of single cat retinal ganglion cells. Stimuli will be generated by a computer-controlled electronic display; neural activity will be monitored with microelectrodes inserted in the optic tract of anaesthetized adult cats. Ganglion cell responses will be interpreted in terms of models of retinal processing that include not only the classical center and surround mechanisms, but also the nonlinear subunit pathway of the Y cell and the contrast gain control. A variety of mathematical techniques, including linear systems analysis and the sum-of-sinusoids method of nolinear systems analysis, will be used to construct and analyze physiological models. A concise and accurate description of retinal processing will serve as a model for understanding of information processing by the local circuits of the mammalian central nervous system. Visual processing in the human cortex will be investigated by studying evoked responses to novel classes of visual stimuli. These stimuli are designed to separate signals due to cortical processing from signals due to precortical mechanisms, and consist of discriminable texture pairs with identical power spectra. Alternation between such textures evoke asymmetric responses which occur with greater latency than the traditional pattern-reversal evoked responses. This technique will be used to examine population properties of cortical visual neurons, such as the role of cooperative interactions and the spatial selectivity of cortical feature-detectors. The possibility that this technique may provide a sensitive probe of abnormalities of cortical function will be explored by studying evoked responses in patients with metabolic and structural neurologic disease.